There is a growing interest in antistatic materials and coatings in various fields of technology, particularly in the photographic and electronics industries. Antistatic materials, i.e., antistats, are electrically conductive materials which are capable of transporting charges away from areas where they are not desired. This conduction process results in the dissipation of electrical charges, i.e., "static electricity." It is desirable to use antistats in applications where it is necessary to avoid the build-up of electrical charges, especially where the static buildup can discharge suddenly and produce a detrimental effect. For example, in imaging systems such as photographic applications, an antistatic coating on the imaging media can help to avoid sudden discharges of built-up electrical charge. A sudden discharge or spark can cause undesirable recordation of the associated flash of light.
A typical antistat comprises an organic or inorganic conductive material in a binder. Of these, preferable antistats are those that conduct electrons by a quantum mechanical mechanism rather than by an ionic mechanism. This is because antistats that conduct electrons by a quantum mechanical mechanism are effective independent of humidity. They are suitable for use under conditions of low relative humidity, without losing effectiveness, and under conditions of high relative humidity, without becoming sticky. A major problem, however, with such electron-conducting antistats is that they generally cannot be provided as thin, transparent, relatively colorless coatings by solution coating methods. Although there have been many attempts to do so, such as by using defect semiconductor oxide particle dispersions and conductive polymers, there has been very little success in overcoming this problem. The use of vanadium oxide has proven to be the one exception. That is, effective antistatic coatings of vanadium oxide can be deposited in transparent, substantially colorless thin films by coating from aqueous dispersions.
Vanadium oxide has three unique properties, i.e., its conduction mechanism, dispersibility, and morphology, which distinguish it from other antistatic coating materials. The latter two properties are generally highly dependent upon the method of synthesis, the first somewhat less so. The conduction mechanism in vanadium oxide is primarily a quantum mechanical mechanism known as small polaron hopping. By this mechanism, electrons are transported through the material by transference (i.e., by "hopping") from one vanadium ion to the next. This conduction mechanism does not require the presence of a well-developed crystalline lattice or a specific defect structure, as do defect semiconductors such as doped tin oxide or doped indium oxide.
Because small polaron hopping electronic conduction does not require a well-developed crystalline structure there is no need for an annealing step when a film or coating is made from vanadium oxide. Furthermore, vanadium oxide is conductive simply upon precipitation or formation in solution, without being adversely affected by changes in relative humidity. Thus, a highly dispersed form of vanadium oxide that exhibits electronic conductivity, and desirable morphology, particle size, and dispersion properties is useful for the preparation of conductive antistatic coatings.
The effectiveness of a dispersed form of vanadium oxide, i.e., a vanadium oxide colloidal dispersion, for the preparation of antistatic coatings can be expressed in terms of the surface concentration of vanadium. The surface concentration is described as the mass of vanadium per unit surface area, i.e., mg of vanadium per m.sup.2 of substrate surface area, required to provide useful electrostatic charge decay rates. Generally, the lower the surface concentration of vanadium needed for effective conductivity in an antistatic coating, the more desirable the vanadium oxide colloidal dispersion. This is because with a lower surface concentration of vanadium, there is typically less color imparted to the coating, the coating is more transparent and uniform, and in some circumstances the coating generally adheres better to the substrate and may even provide better adhesion for subsequent layers.
In the mid-1970's, Claude Guestaux of Eastman Kodak reported that a previously known synthetic method provides a vanadium oxide colloidal dispersion which, at the time, was considered uniquely useful for the preparation of antistatic coatings. Guestaux's method was based on a process originally described by Maller in 1911. The method is described in U.S. Pat. No. 4,203,769 and consists of pouring molten vanadium pentoxide into water. The process produces a good antistatic coating, but has several drawbacks. These drawbacks include high energy requirements, the need for special reactor materials and equipment, and the creation of conditions which generate toxic vanadium oxide fumes. Furthermore, the Guestaux method results in incomplete dispersion of vanadium oxide. The nondispersed vanadium oxide must then be removed from the viscous dispersion. However, such viscous vanadium oxide dispersions are usually very difficult to filter.
There are several other methods known for the preparation of vanadium oxide colloidal dispersions. These include inorganic methods such as ion exchange acidification Of NaVO.sub.3, thermohydrolysis of VOCl.sub.3, and reaction of V.sub.2 O.sub.5 with H.sub.2 O.sub.2. However, vanadium oxide colloidal dispersions prepared by these methods using inorganic precursors are much less effective for the preparation of antistatic coatings than colloidal dispersions prepared by the process described by Guestaux in U.S. Pat. No. 4,203,769. To provide coatings with effective antistatic properties from dispersions prepared from inorganic precursors typically requires substantial surface concentrations of vanadium. Higher surface concentrations of vanadium generally result in the loss of desirable properties such as transparency, adhesion, and uniformity.
Flexography is a term that broadly applies to a printing format using a flexible substrate bearing an elastomeric or rubbery relief printing surface.
The first flexographic printing plates were produced from natural or synthetic rubber compositions which were cured chemically under heat and pressure in a mold utilizing conventional rubber curatives such as mercapto compounds (Flexography: Principles and Practices, 3rd Edition, Flexographic Technical Association, p. 158-162). More recently, photopolymer elastomeric compositions (elastomer containing compositions curable upon exposure to actinic radiation) useful to produce relief printing plates have been described. For example, U.S. Pat. Nos. 4,369,246 and 4,423,135 describe solvent-insoluble, elastomeric printing relief plates which are formed by applying to a sheet support a layer of a photosensitive composition comprising (1) at least 30 weight % of a solvent-soluble co-polymer containing at least 2 thermoplastic, non-elastomeric blocks of glass transition temperature above 25.degree. C. and average molecular weight 2000-100000 and between these blocks, an elastomeric block copolymer having a glass transition temperature below 10.degree. C. and average molecular weight 25,000-1,000,000; (2) at least 1 weight % of an addition polymerizable compound containing at least one terminal ethylenic group; and (3) a polymerization initiator at a dry thickness of 0.005-0.250 inch. A flexible polymer film and flexible cover sheet are laminated to the composition layer. The plate is formed by stripping off the cover sheet, imagewise exposing the layer to actinic radiation through the film, and removing the film and unexposed areas of the layer by solvent washing. Solvents such as perchloroethylene (1,1,1 trichloroethylene) either alone or in combination with alcohols such as n-butanol are utlized. Likewise, EP Pat. 261,910 describes a further example of a water-developable flexographic relief printing plate comprised of (1) monomers and polymers of acrylic acid esters and (2) a ketone photopolymerizing/photocrosslinking agent, which are coated on a support sheet. Following imagewise exposure (to promote crosslinking), the relief areas of the plate are produced by washout with an aqueous developer. After washout, all of the flexographic platemaking compositions and methods described heretofore require drying of the plate for extended periods (I to 24 hours) to remove entrained developer solution and then are subjected to a post-finishing process (chemical or photochemical) to reduce surface tack of the plate before use on a printing press. In addition to the extended time periods required to produce flexographic printing plates by the aforementioned technologies (by reason of the multiplicity of processing steps required in series), these technologies also produce potentially toxic by-product wastes in the development procedure. In the case of the solvent-washout technology, organic solvent waste is generated which is potentially toxic in the form of both the solvent and the addition polymerizable compound 2) which contains at least one terminal ethylenic group. Likewise,, the aqueous washout plate technologies also produce a contaminated waste water by-product stream which may contain similar addition polymerizable compounds that may have cytotoxic effects.
Many different types of monomers and cross-linkable resins are known in the polymer art, their properties can be adjusted as taught in the art to provide rigidity, flexibility, or other properties. Particularly good materials related to the compositions of the present invention are shown in U.S. Pat. Nos. 4,578,504; 4,638,040; and 4,786,657.
Various types of curable polyurethane elastomeric compositions are known. The term "elastomer" or "elastomeric" is used to refer to rubbers or polymers that have resiliency properties similar to those of rubber. In particular, the term elastomer reflects the property of the material that it can undergo a substantial elongation and then return to its original dimensions upon release of the stress elongating the elastomer. In all cases an elastomer must be able to undergo at least 10% elongation (at a thickness of 0.5 mm) and return to its original dimensions after being held at that elongation for 2 seconds and after being allowed 1 minute relaxation time. More typically an elastomer can undergo 25% elongation without exceeding its elastic limit. In some cases elastomers can undergo elongation to as much as 300% or more of its original dimensions without tearing or exceeding the elastic limit of the composition. Elastomers are typically defined to reflect this elasticity as in ASTM Designation D883-866 as a macro-molecular material that at room temperature returns rapidly to approximately its initial dimensions and shape after substantial deformation by a weak stress and release of the stress. ASTM Designation D412-87 can be an appropriate procedure for testing rubber properties in tension to evaluate elastomeric properties. Generally, such compositions include relatively high molecular weight compounds which, upon curing, form an integrated network or structure. The curing may be by a variety of means, including: through the use of chemical curing agents, catalysts, and/or irradiation. The final physical properties of the cured material are a function of a variety of factors, most notably: number and weight average polymer molecular weights; the melting or softening point of the reinforcing domains (hard segment) of the elastomer (which, for example, can be determined according to ASTM Designation D1238-86); the percent by weight of the elastomer composition which comprises the hard segment domains; the structure of the toughening or soft segment (low Tg) portion of the elastomer composition; the cross-link density (average molecular weight between crosslinks); and the nature and levels of additives or adjuvants, etc. The term "cured", as used herein, means cross-linked or chemically transformed to a thermoset (non-melting) or relatively insoluble condition.
U.S. Pat. No. 4,939,008 describes an aziridine functional layer which is used to bond a polyolefin layer to a polymeric substrate. The aziridine functional material is the same as the materials of the present invention used as a primer.
U.S. Pat. No. 5,015,446 describes a novel process for developing and imaging a flexographic printing plate in which backside ionizing radiation is used to establish a floor in the printing plate. The plate may then be dry developed by absorbing non-imagewise-irradiated composition from the printing plate. This type of plate offers many benefits to the user and the environment, but like many imaging materials suffers from static problems, especially in transport equipment. The static not only affects imaging, but may also impede the physical transport of materials through the developing apparatus.
U.S. patent application Ser. No. 07/699,666, filed on May 14, 1991 in the name of Prioleau and Canty discloses a preferred aziridine priming system for use with flexographic printing plates, especially those of the type described in U.S. Pat. No. 5,015,446.